(1) Field of the Invention
The present invention relates to an exhaust emission control apparatus, and more particularly to a NOx occluding catalyst which purifies NOx with a high efficiency.
(2) Description of the Related Art
In recent years, a lean burn internal combustion engine which is capable of performing combustion in an oxygen-excess atmosphere (at a lean air-fuel ratio) has been put into practical use so as to improve fuel economy. The lean burn internal combustion engine (lean burn engine) employs a NOx occluding catalyst which purifies NOx in exhaust gas by occluding NOx during lean burn. It has been known that the NOx occluding catalyst has a function of purifying NOx in exhaust gas by occluding NOx onto the catalyst in an oxygen-excess atmosphere (at a lean air-fuel ratio), and emitting the occluded NOx when the oxygen concentration lowers (at a stoichiometric or rich air-fuel ratio). Specifically, in an oxygen-excess atmosphere, the NOx occluding catalyst generates nitrate from NOx in exhaust gas to occlude NOx, and on the other hand, in an oxygen concentration reduced atmosphere, the NOx occluding catalyst causes the nitrate occluded to the NOx occluding catalyst and CO in exhaust gas to react with each other to generate carbonate so that NOx can be emitted.
By the way, the NOx occluding catalyst occludes NOx onto the catalyst in an oxygen-excess atmosphere during lean burn, but when the amount of occluded NOx reaches a saturation amount as a result of continuous lean burn, NOx in exhaust gas cannot be occluded and emitted into the air. Thus, before the amount of occluded NOx reaches a saturation amount, the air-fuel ratio of the lean burn engine is switched to a stoichiometric or rich air-fuel ratio to change exhaust gas into an atmosphere having a low oxygen concentration, so that a large amount of reducing agents such as CO and HC are generated to emit and reduce NOx to restore the NOx occluding capability of the NOx occluding catalyst (this is called “NOx purge”).
In this operational range, in the case where NOx is emitted and reduced with the air-fuel ratio of the lean burn engine being switched to a stoichiometric or rich air-fuel ratio so as to restore the NOx occluding capability, and part of the supplied reducing agents such as CO and HC is consumed so as to emit the occluded NOx, and the remaining reducing agents are consumed so as to reduce the emitted NOx.
In the NOx purge, however, if the amount of reducing agents such as CO and HC is small, NOx occluded to the NOx occluding catalyst cannot be satisfactorily emitted and reduced, which deteriorates not only the NOx occluding performance during operation at a lean air-fuel ratio but also the fuel economy due to an increase in the frequency of regeneration. Therefore, if the air-fuel ratio is made considerably richer as compared with a stoichiometric air-fuel ratio so that NOx occluded to the NOx occluding catalyst can be satisfactorily emitted and reduced, i.e. the amount of reducing agents such as CO and HC can be sufficient, reducing agents such as CO and HC which are not used for emission and reduction are emitted to an area downstream of the NOx occluding catalyst.
To address this problem, it is known that a three-way catalyst is disposed downstream of the NOx occluding catalyst so as to remove NOx, CO, and HC (Patent Publication 1: JP 11-002144 A). An exhaust emission control apparatus disclosed in the Patent Publication 1 is constructed such that a front three-way catalyst, a NOx catalyst, and a rear three-way catalyst are arranged in cascade in this order.
For example, a direct-injection lean burn engine needs to maintain a high NOx trapping capability during operation at a lean air-fuel ratio and a high HC and CO purifying capability during operation at a stoichiometric or rich air-fuel ratio. Therefore, as shown in FIG. 12A, an exhaust emission control apparatus employing a tandem system is used in which a NOx catalyst 100 and a three-way catalyst 110 are arranged in cascade along an exhaust passage.
Also, as shown in FIG. 12B, an exhaust emission control apparatus employing a single system is used in which a single ceramic carrier is disposed along an exhaust passage, and a NOx catalyst 100′ and noble metal 110′ are supported on the carrier so as to ensure a NOx trapping capability and a three-way catalyst capability.
In the exhaust emission control apparatus as shown in FIG. 12A and disclosed in the patent publication 1, the number of catalysts and the catalyst capacity are increased, and this increases exhaust pressure loss and reduces engine power. On the other hand, in the exhaust emission control apparatus constituting the single system as shown in FIG. 12B, although an increase in catalyst capacity and exhaust pressure loss can be suppressed, NOx trapping agents and noble metal operate in such a manner as to suppress the functions of each other, and hence optimum NOx trapping performance and three-way catalyst performance cannot be achieved.
Further, in the exhaust emission control apparatus, it can be envisaged that the amount of noble metal supported on the catalyst is increased so as to improve the HC and CO purifying performance and the NOx purifying performance during operation at a lean air-fuel ratio, but this is encountered with the problem that the NOx purifying performance is actually degraded. This is assumed to be because a phenomenon described below occurs for the reason that the consumption of reducing agents during operation at a rich air-fuel ratio is increased due to an improvement in oxidizing performance as a result of an increase in the amount of supported noble metal.
Firstly, the amount of reducing agents for NOx emitted from the NOx occluding agent is insufficient.
Secondly, when the air-fuel ratio is rich immediately after the air-fuel ratio is switched from a lean air-fuel ratio to a rich air-fuel ratio, it is difficult to restore the activity of noble metal due to oxygen poisoning caused by the shortage of reducing agents (the activity of noble metal is kept degraded).
Thirdly, when the air-fuel ratio is rich, the emission of NOx occluded to the NOx trapping agents is suppressed due to the shortage of reducing agents, and hence the NOx purge cannot be satisfactorily performed.